Octanuclearity in copper(II) chemistry: Preparation, characterization, and magnetochemistry of [Cu-8(dpk center dot OH)(8)(O2CCH3)(4)](ClO4)(4)center dot 9H(2)O (dpk center dot H2O equals the hydrated, gem-diol form of Di-2-pyridyi ketone)

The complexes [Cu-8(dpk . OH)(8)(O2CMe)(4)](ClO4)(4) . 9H(2)O (1) and [Cu(dpk . H2O)(2)](O2CMe)(ClO4). 2H(2)O (2), where dpk . H2O is the hydrated, gem-diol form of di-2-pyridyl ketone, have been prepared. Complex I crystallizes in triclinic space group P (1) over bar with the following unit cell dimensions at 25 degrees C: a = 18.396(1) Angstrom, b = 16.720(1) Angstrom, c = 19.171(1) Angstrom, alpha = 96.10(1)degrees, beta = 87.68(1)degrees, gamma = 99.14(1)degrees, Z = 2. Crystal structure data for 2 al room temperature are as follows: monoclinic, P2(1)/c, a = 13.000(2) Angstrom, b = 8.008(1) Angstrom, c = 27.095(3) Angstrom, beta = 93.19(1)degrees, Z = 4. The two centrosymmetrically related cubanes in the tetracation of 1 are doubly-bridged with two syn, anti acetate groups bridging two Cu-II atoms. The monoanion dpk . OH- functions as a eta(1):eta(3):eta(1):mu(3) ligand, Three Cu-II atoms have distorted octahedral coordination geometries with CuO3N3 and CuNO5 chromophores, while the fourth Cu-II center displays a distorted square pyramidal geometry; a terminal monodentate acetate is Ligated to this latter Cu-II atom. In the mononuclear [Cu(dpk . H2O)(2)](2+) cation of 2, the four pyridyl nitrogens can be viewed as strongly coordinating to the metal (Cu-N = 2.013(4)-2.022(4) Angstrom), while one of the hydroxyl oxygens on each ligand forms a weak bond to Cu-II (Cu-O=2.417(4), 2.352(3) Angstrom). Variable-temperature magnetic susceptibility studies on 1 are in line with both an overall antiferromagnetic interaction between Cu-II atoms and the magnetic behavior of a simple cubane. Exchange parameters, J, derived by using a four-J magnetic model, are found to be J(1) = 6 cm(-1), J(2) = -144 cm(-1), J(3) = -14 cm(-1), J(4) = 3 cm(-1) and g 2.29 (adjustable parameter) by least-squares fitting to the spin Hamiltonian H = -2 Sigma(i<j)J(ij)S(i) . S-j. The thus derived energy level spectrum shows a S = 1 ground state further supported by the solid-stare and solution EPR spectra of 1. insight concerning the effect of structural parameters on the magnitude of the magnetic exchange interactions was gained through EHMO calculations performed on a model Cu(OR)(2)Cu moiety. Accordingly, estimates of the J parameters, experimentally derived, were in close agreement both with known magneto-structural correlations established for planar Cu(OR)(2)Cu moieties and a criterion established by us, holding for the magneto-structural correlations in symmetrical roof-shaped, alkoxo-bridged Cu(OR)(2)Cu moieties.